1. Field of the Invention
The invention relates to a process for the preparation of formulations comprising bismuth salts, where bismuth oxide is reacted in a specific way with lactic acid and then the resulting mixture of bismuthyl lactate and bismuth lactate is dispersed in a coating binder, and to the use of these formulations as catalyst component in cathodic electrodeposition coating materials.
2. Description of Related Art
AT 397.820 B1 discloses a process for the preparation of catalyst-containing cationic coating binders which are water-dilutable after protonation, and which comprise cationic coating binders crosslinkable by transesterification and/or transamidation and/or transurethanization and/or by reaction of terminal double bonds, and bismuth salts of aliphatic hydroxycarboxylic acids, preferably of lactic acid or dimethylolpropionic acid.
According to this proposal, the bismuth salts are obtained by the reaction of 1.0 mol of bismuth oxide with 7.0 mol of the respective hydroxycarboxylic acid, the excess of 1.0 mol of acid being intended to guarantee as quantitative a reaction as possible. Deionized water and the acid are the initial charge, and commercial bismuth oxide (Bi.sub.2 O.sub.3) is added in portions at 70.degree. C. with stirring. The precipitate is filtered off and dried at a temperature of from 40 to 60.degree. C.
The catalyst-containing coating binders are prepared by adding the bismuth salt of the protonated binder solution, prior to the addition of substantial quantities of water as diluent, and then homogenizing the mixture for a number of hours.
According to WO 93/24578, claims 3 and 5, the bismuth salt is homogenized in a high-speed mixer, in a coating binder which can be used as paste resin, and then in a bead mill, if desired in the presence of pigments.
The processes described in the two patent documents exhibit a number of substantial disadvantages in their industrial applicability.
On the one hand, the isolated and dried bismuth salts have a tendency while being stored to undergo agglomeration. On the other hand, the quantity of acid required for the "digestion" of the bismuth oxide is greater than that which is subsequently required for neutralizing the overall coating binder. Consequently, excess acid is introduced into the deposition coating material, thereby considerably increasing the amount of current required during the operation of the deposition tank.
The specific embodiment which is described in both patent documents for the case where lactic acid or dimethylolpropionic acid is used as neutralizing agent for the overall binder, of homogenizing not bismuth salt but, wholly or partially, the corresponding quantities of bismuth oxide or bismuth hydroxide--in this in situ method it is not necessary to isolate the bismuth salts--is likewise only of conditional success. Coating materials formulated correspondingly have in many cases a tendency to form a sediment after prolonged storage, and furthermore a reduction in the catalytic activity of the bismuth salts is observed.
If, in contrast, the quantity of acid is reduced to less than 7.0 mol per 1.0 mol of bismuth oxide, then during the "digestion" the reaction mixture frequently forms a tacky crystalline mass which can neither be stirred nor filtered.
Even the use of a paste resin in accordance with WO 93/23578 does nothing to alter the difficulties indicated.